This invention relates generally to xerographic copying machines, and more particularly to a copier capable of producing copies in a selected magnification ratio with respect to the original document from which the copies are produced.
In the xerographic technique, a photoconductive insulating layer whose surface is uniformly charged electrically is first exposed to an illuminated pattern of light and shadow of the intelligence to be recorded. The blanket charge on the layer is selectively dissipated by the illuminated pattern to yield a latent electrostatic image. Thereafter, to develop the image, finely-divided pigmented thermoplastic powder or toner is deposited on the latent image, the toner particles adhering to the electrostatically-charged areas in proportion to the charges thereon.
In a plain paper xerographic printer, the photoconductive insulating layer is supported on a rotating drum or on a continuous belt and the toner image developed on the surface of this layer is transferred therefrom onto a sheet of ordinary paper. The developed image on the paper is then fixed thereto by heat or pressure which fuses the toner particles to the paper.
In a treated-paper xerographic printer, there is no need to transfer the developed toner image from the photoconductive insulating layer, for in this instance use is made of paper coated with photoconductive zinc oxide particles dispersed in a film-forming resin binder. The coated surface of the paper is subjected to a blanket electrostatic charge which is then exposed to the light pattern to be recorded to create a latent image thereon. This latent image is developed by toner which is directly fixed onto the treated paper, thereby obviating the transfer step characteristic of an untreated paper printer.
The present invention is concerned primarily with apparatus adapted selectively to change the magnification ratio of the copy with respect to the original document, the invention being fully applicable both to treated and plain paper xerographic copier machines.
Electrostatic copiers are known which are capable of producing copies that may be either full-scale copies of the original document or enlarged or reduced in size with respect to the original document. Thus in the Lux U.S. Pat. No. 3,556,655, there is disclosed for this purpose a turret lens assembly movable between different positions for projecting a full-size or a reduced-size image of an original onto a a copy sheet.
To avoid the need for employing different magnifying lens for selectively changing the magnification ratio, the Reehil et al. U.S. Pat. No. 3,778,147 provides a single lens which is made linearly movable with respect to the original document and an image plane. In a similar fashion, in the Knechtel U.S. Pat. No. 3,703,334, a change in magnification is effected by shifting the position of an objective lens and of the mirror associated therewith. Reproductions of different scale are likewise effected in the Muller U.S. Pat. No. 3,687,544 by shifting the position of an objective and its associated mirror.
Thus in order to change the magnification ratio in an electrostatic copier machine, it was heretofore the practice to change the distance between the original document and the objective lens as well as the position of mirrors associated with the lens. These requirements introduce mechanical problems which add substantially to the cost and complexity of the machine. Moreover, the space heretofore needed to incorporate a selectable magnification ratio system into a standard copier is such as to expand the machine dimensions, further adding to the cost of manufacture and precluding a compact structure.